The present invention relates to a reading circuit suitable for use with an optical disk apparatus or similar optical information storing apparatus and, more particularly, to a reading circuit of the type producing read out data in the form of a digital binary signal be detecting peaks of an analog signal, or information read out of a recording medium, by a stored information detecting device.
In an optical storing appartatus such as an optical disk apparatus, a storing medium implemented as an optical disk is illuminated by a laser beam which is regulated into a small spot, and light relfected from the disk is changed in intensity depending upon the present/absence of a pit, or stored information, on the disk surface. An optical sensor is installed in the apparatus to sense such a change in the intensity of reflection to produce an analog signal. This analog signal has a waveform which contains information at its peaks which are individually associated with the centers of the pits on the disk. Hence, use is made of a reading cirucit which is so constructed as to produce read out data in the form of a digital signal by detecting the peaks of the analog signal.
Typical of implementations for detecting the peaks heretofore proposesd is the combination of a differentiating circuit and a zero-crossing comparing circuit. The differentiating circuit differentiates the analog signal while the zero-crossing comparing circuit detects the points where the differentiated analog signal crosses the zero level. A drawback with such an implementation is that high-frequency noise increases due to particular frequency characteristics of the differentiating circuit and, in addition, the signal-to-noise (S/N) ratio of an analog signal of the kind described is limited. In light of this, the reading circuit is also provided with a level detecting function adapted to check the amplitude of the signal for thereby eliminating erroneous detection of data due to baseline noise. More specifically, the reading circuit produces read out data on the basis of coincidence of a peak detection output and a level detection output. A conventional approach for such level detection is differentiating the analog signal in two consecutive steps and comparing the level of the resulting signal with a pedetemined threshold voltage level. Such an approach is substantially immune to the influence of envelope fluctuations ascribable to low frequency components contained in the analog signal and implements the level detection by a relatively simple circuit construction.
However, the prior art scheme for level detection discussed above has a drawback that since it is the analog signal undergone a double differentiation that is used for the detection of levels, high-frequency noise is increased beyond the signal undergone a single differentiation for peak detection due to the particular characteristics of the differentiating circuit. Consesquently, data ONEs are apt to be erroneously detected at ZERO data positions where no pits exist, owing to the baseline noise. Erroneous detection due to noise is further aggravated because the double differentiated signal is used in undershoot waveform portions of the analog signal which are derived from the diffraction effect particular to pits and/or from the characteristic of a low-pass filter included in an amplifying circuit.